When a nuclear bomb is detonated, intense fluxes of gamma rays, x-rays, and other high energy particles are created. When integrated circuits are exposed to this type of harsh environment, a large number of electrons and holes are generated in the silicon, causing large photocurrents to be generated. Under certain conditions, these photocurrents can lead to rail-span collapse, and burnout of metal lines, contacts, and vias. This damage to the integrated circuits can ultimately result in system failure.
Some integrated circuits, such as integrated circuits used in strategic weapons systems, are designed to continue operating during and after a nuclear bomb attack or other dose rate event. Many of these integrated circuits include memory. The memory exposed to dose rate events needs to be hardened or the memory may be damaged.
MRAM is one type of memory that may be used in applications that expose circuitry to dose rate events. Typically, each memory cell in MRAM contains a magnetic tunnel junction (MTJ) structure and a selection or control transistor. The MTJ structure consists of a layer of insulating material between two electrodes of magnetic material. Current flows or “tunnels” perpendicularly from one magnetic layer to the other magnetic layer through the insulator. At the base of one electrode is a fixed ferromagnetic layer that creates a strong pinning field to hold the magnetic polarization of the layer in one specific direction. The other ferromagnetic layer is free to rotate and hold polarization in one of the two directions.
The MTJ structure is sensitive to bias voltage placed across it. During a dose rate event, photocurrent flows through the MTJ structure, which can damage the insulating material in the MTJ structure. For example, voltages in excess of 500 mV may cause significant reliability degradation, while voltages in the 1 V range may cause catastrophic damage, usually in the form of a short circuit condition. Therefore, it would be beneficial to harden the MTJ structure to protect MRAM bits during a dose rate event.